1. Field of the Invention
The present invention relates to a power conversion apparatus which is able to adjust voltage of an electric power system for controlling voltage of a load connected to the power system thereby achieving a power-saving effect.
2. Description of the Related Art
In a prior art power conversion apparatus, for example, in a single-phase power supply, a method is used in which an autotransformer is provided with a plurality of taps.
Here, as a first prior art, a circuit configuration of power-saving apparatus is shown in FIG. 11. In the circuit shown in FIG. 11, an inputted AC voltage at single-phase AC input terminals 1A and 1B is adjusted by changing over a tap 40 and outputted from single-phase AC output terminals 5C and 5D.
As shown above, since voltage control to the load is performed by tap change over, voltage control can be performed only at voltage levels of the number of previously provided taps, therefore, when a fine control is required, the number of taps has to be increased.
FIG. 12 shows a second prior art power-saving apparatus example which can solve the problem with the tap change-over control.
In the circuit shown in FIG. 12, the AC voltage inputted between the single-phase AC input terminals 1A and 1B is converted to a voltage of the same phase and differing in amplitude by a voltage amplitude adjuster 21 composed of semiconductor switches, then removed of ripples by an AC filter 25, and outputted from the single-phase AC output terminals 5C and 5D.
As shown above, AC voltage is controlled by ON/OFF operation of the semiconductor switches. Therefore, the AC voltage can always be maintained at a preset voltage.
Since, in the first prior art, voltage control has been performed by tap change over, there remain the following problems.
(1) Since the control capacity of output voltage is determined by the number of taps, to improve the accuracy of control, the number of taps is increased which results in a large-sized apparatus and a cost increase. PA0 (2) When the taps are composed of mechanical contacts, it is necessary to consider the problem of service lives due to wear of the contacts, and an appropriate maintenance is required. Further, an instantaneous interruption or the like may occur during tap change over. PA0 (3) Even if the tap is substituted with a thyristor, there still remains a problem of flicker or the like. PA0 (4) a capacity of the conversion apparatus corresponding to the load power is required, which deteriorates the efficiency compared to the first prior art method, therefore, the desired power-saving effect can not be obtained.
Further, since, in the second prior art, continuous voltage control by the semiconductor switches can be performed, the problems with voltage control by the tap change-over method can be solved, however, there is a problem that